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TIDA-010054: What is the minimum phase shift

abhishek deshmukh
Prodigy 220 points
Part Number: TIDA-010054

Hello,

We are referring the ref design of DAB topology TIDA-010054.

Here the switching freq. is 100KHz & max, output power = 10kW.

In single phase shift method, the max. phase shift is of 90 degrees.

Now ideally, 90 degrees will correspond to 5 us time duration (turn ON time, excluding dead time) & 10kW of output power. 

So, 1 degree will correspond to 55.55 ns & 111.11W of output power.

Are the above calculations, correct?

In the eval. board, how much is the minimum phase shift that can be achieved (i.e., min. time delay between primary & secondary bridges) & min. power that can be delivered?

Regards,

Abhishek D.

  • +1
    TI__Expert 4400 points

    Hi Abhishek,

    Thanks for reaching out.  It is correct, that in a DAB the maximum power transfer happens at 90 degrees of phase shift. In our TIDA-010054 the phase shift is limited to 23 degrees actually. This is enough to transfer 10 kw at 800V Vin and 500V Vout. The reason is that the relation between phase shift and transferred power is not linear, but at higher phase shift values the delta in the power over the delta in the Phase shift get smaller, so we use the range up to 23 degrees where the relation is closer to linear.

    So your calculations are not correct here. Refer to equation 6 in the design guide for the calculation of the power transfer, with n= PrimaryTurns/SecondaryTurns  of the transformer = 1.6, V1 = 800V, V2 = 500V Phi = 23 degree = 0,401426rad, Fs - 100kHz and L =35 uH:

    The minimum phase shift  depends on the resolution of the PWM  which is 150 ps for the used High resolution PWM module.

    For the nominal input and output voltages the load can be reduced down to the 100s of Watts range, that's not a problem, but for if the relation between input and output voltage varies, There can be a loss of ZVS which reduces the efficiency drastically.

    I hope this is helpful for you.

    Best regards,

    Andreas

  • 0 in reply to Andreas Lechner
    Prodigy 220 points

    Thank you, Andrea Lechner.

    But say if load is 200W then how much min. phase shift is required in this design?

    I am working on 11kW onboard charger & designing the leakage inductor with the above formula (mentioned in ref. design) considering 75 degrees phase shift for max o//p power & inductor value of 40uH.

    I kept 15 degrees phase shift margin (90 - 75 =15)

    I have also simulated the DAB topology & phase shift in simulation & calculations are nearby. 

    I kept phase shift to higher degree in order to meet light load condition with some achievable (feasible) phase shift value. 

    Is this approach correct or do we need to also consider phase shift in the range of 23 degrees as in Texas ref. design.

    Regards,

    Abhishek D.

  • +1 in reply to abhishek deshmukh
    TI__Expert 4400 points

    Hi Abhishek,

    It is not a must to limit the phase shift to 23 degrees, but it makes the design of the controller easier. 
    You can solve the same equation for the phase shift and insert 200W. 

    This is done in equation 14:

    For our design the phase shift is 0.395 degrees for 200W in theory. This is not a problem regarding the resolution of the PWM, but in a real application factors like dead time will have an influence as well.


    We did not aim our design for these really light load conditions, so it  might be better to go or a higher inductance like you mentioned.

    Best regards,
    Andreas